The Concept of Sound Pressure (SPL)

Introduction: The Concept of Sound Pressure (SPL)

Have you ever been told to turn the volume down on your music? Have you ever wondered why when something is loud it is said to have more volume? In this instructable I hope to clear up some major misconceptions about sound, and hopefully help you understand how sound works.

This instructable is more of a how it works as opposed to a how to. Once you understand how it works, it is much easier to learn the how to.

Please note that I am constructing this from a sound engineer's point of view. Most of the content is theory, so please keep that in mind when you read this.

If you haven't read my first instructable "How Sound Works" yet, I suggest you read it first. You can find it here: https://www.instructables.com/id/How-Sound-Works/

Step 1: What Is Sound Pressure?

Sound Pressure Level is normally abbreviated SPL. SPL is calculated in decibels or db. Please note that db is a relative term. It can refer to almost anything, however, the most well known is SPL. Most of the time when someone is talking about db, they are probably referring to SPL.

Step 2: The Difference Between Sound Pressure and Volume

The human ear hears pressure, not volume. The difference between sound pressure and volume is that the closer that to the source of the sound, the more pressure you will hear. The farther away you get from the source of the sound, the pressure will be less, but the volume will be the same. A good analogy would be a faucet. At the top of the faucet where the water comes out, there is more pressure than at the bottom, but the volume of water is the same. (See picture)

One thing to remember with SPL is that as you turn the pressure up, you generally also increase the volume. Volume does affect sound, even though it does not directly affect how we perceive how loud something is. One of the main characteristics of a professional speaker is that it can produce more pressure with less volume than a consumer speaker can.

Step 3: Why Is It Important?

SPL can also damage the ear. The human ear starts to degenerate at an SPL of about 85-90 db. You can buy SPL meters at almost any store that sells professional audio equipment, or radio shack. Because a standard rock concert has an approximate SPL of 100-130 db, if you are mixing for any event, you should keep a SPL meter handy and try to keep the SPL at or below 80 db. If you are attending a rock concert, you should make sure to have hearing protection with you. You can buy discrete hearing protection at almost any department store or store that sells professional audio equipment.

Step 4: The Inverse Square Law

One thing to remember when you are mixing, is that in normal conditions, when you double you distance from the source of the sound, you reduce the sound by 6 db (4X). So if you are in the back of the room and your speakers are at the front, the pressure at the front of the room is much more at the front than at the back.

Step 5: Conclusion

If you only take one thing away from this instructable, it should be a warning not to ruin anyone else's hearing, and to protect your own. If our generation continues to attend loud rock concerts, then in 20 years, 10 year old kids will be using hearing aids.

Step 6: Copyright

Please note that I do claim copyright to the information. I did not use any specific sources when compiling this information, all of this is from my personal experience.

You may quote parts of this information for educational purposes. Under no circumstances will you sell this information.

I do not own the copyright to any of the images (except the picture in step #2), however, as far as I have been able to find, I have the right to use them in this instructable. If there is any question about whether or not I have the right to use these images, please contact me. I have no intention of stealing anyone's intellectual property.

Use of this information implies that you agree to these copyright terms.

Comments

hi..i wander what is the SPL relation to frequence? at the same given electrical power 60 Hz would have the same pressure of 8000 Hz? in pounds? it doesn't seems by the air coming out of a speaker cabinet.. at this point i ask why the word pressure is there at all..?Ears drum hurt at the pressure of water and maybe because of to much acoustic power from a spk.but i am so tempted in thinking that the pain of high frequence is more an issue of meccanical vibrations.. what you think?

It explains how frequency (pitch) of a sound relates to volume (the Sound Pressure level measured in dB) and loudness (subjective/something we perceive).

Basically, a lower-frequency sound needs to have more SPL(volume/power/amplitude) to be perceived as having the same loudnessas a higher-frequency sound. So, for example, if you have a perfect sine wave playing a low C, and then another perfect sine wave playing a C three octaves above that, you'll need to have the lower-frequency (low C) of the two sine waves playing at a higher SPL/dB/amplitude in order for both the sounds to seem equally as loud to the human listening to them.

For the second part of your question about pressure level... (I looked this up, but I'm glad I did because now I know! =] ) The reason it doesn't seem like "pressure" at all is because it's so much smaller than anything you compare it to for feeling pressure (like pressure on your ears diving underwater, for example). To explain: volume is objectively measured in dB, which is a logarithmic scale based on how much more volume (SPL) a sound has relative to the quietest limit of human hearing. But the basic physical unit for measuring SPL itself is actually: the Pascal. According to the Internet, 1 Pascal = 0.000145038 PSI(pounds per square inch). So yes... it is pressure, the exact same physical phenomenon of having an additional atmosphere's worth of pressure on you (14.6959 pounds per square inch) when you dive 10 meters down underwater - it's just with sound, the differences in pressure are so small we don't actually feel them as pressure, we just hear and perceive those differences as "louder" or "quieter" with the precise sensitivity of our ears.

For the last thing you mentioned, you can definitely have a high-frequency sound (high-pitched) without it being damaging to the ears (which you probably know - just remember that frequency is pitch, not volume). It's when the volume or loudness is too much that damage occurs. I would be interested to know if damage to our ears is worse based on a combination of amplitude and frequency, or if it's just based on SPL across the frequency spectrum. (If someone knows the answer to this, please post it with a link!) Also can sounds damage our ears if we can't even hear them? I want to know and do everything I can to protect my hearing for life.

Just don't destroy anyone's hearing! ;) The thing I don't like about professional audio techs mixing rock concerts is that they seem like they have more pressure than they actually do (there are ways to do that with the EQ), so the amateur techs think that they just have to turn everything up and it will sound professional... little do they know they are destroying everyone's hearing! (That's my little rant and rave... forgive me!) :)

I do think most concerts are way too loud but it's worse when the gear can't handle it and it sounds terrible!I think most music is too loud today. I'd rather ditch my Logitech 40 watt speaker system for a 3 watt a channel tube amp and decent speakers any day!

I do understand where he's coming from... As someone who does AV as a side job, I do fuss a lot with copyrights, and I doubt he would have made a big deal about it if I didn't put the copyright step in. Also, when I first started publishing ibles, I didn't think of specifically stating that I didn't claim copyright to the images, so it made it seem as if I did claim copyright to them. I am grateful to a few ibles members (I think kiteman was one of them) who originally pointed that out. :)

Those images are stock photos. They were found on public sights with public domain. (As far as I could tell.) (All the photos except the sink... which I took myself) And although copyright is covered under the license, until I published an instructable I never realized that there even was a usage license. Now because I made a point of it, there isn't any excuse.

Regarding the shuttle-scale... yes... I was aware of that, I believe that anything over 200 db SPL is fatal. (I remember being told that at one point... I have never actually tested it out...) ;)

First of all, I didn't get that image from that site. The site I got it from had no mention of copyright. Secondly, I saw copyright claimed for that site, but not for the image. More than likely they got it from the same place I did. That image is pretty common. Also, I got that image 5 years ago... which predates that copyright. Although I only used it in an instructable about a month ago when I published my first instructable on sound, I have used that image for years. (Granted, I haven't had all of those images that long, but it is true for the first image.)

I have no intention of stealing intellectual property, so if you can send me a link or screen shot of the place where you are seeing a valid copyright on any of those images, I will gladly take a picture myself to replace it. :)

Which stock-image site - point us please. Please appreciate that without citing your image sources, in stamping step 6 on this you are effectively trying to claim the image as your intellectual property.

But unless you've published an instructable, you probably wouldn't know that there even is a license on instructables... I know I didn't. Do you agree with that, or do you think pretty much everyone does know about that?

If you're responsible enough to use the internet, you should be aware that all information, data and images you see online automatically belong to somebody else, and it is a breach of copyright to use them without first checking the status of the license.

The US school system is so messed up that the kids in public school these days are probably taught how to illeagly download music in their computer class! And then their parents wonder why their public schooled kids can't even get their foot in the door at college, but homeschoolers are accepted to whatever college they apply to!

Do you still think I should remove it after the edit I made to it? I'm not aware of a "standard" distance to measure... at least not for pro-audio. You would generally take measurements in a lot of spots in the area you are mixing in. Although the inverse square rule is good to know if you're short on time, I never rely on it if I can avoid it. Sound is effected by EVERYTHING. (Such as how many people are in a room, the humidity, tempature, just to name a few!) Therefore, I am constantly training volenteers on how to take SPL readings... and having them taking readings on every night. Also, it will give indications on how to position your speakers. I have the ability to visulize how sound waves will interact with the enviorment, but it's always nice to double check it with a meter.

Regarding bringing a meter to a party, there is a simple way to deal with that: use it discreatly, and just say no if someone wants to play with it. It's a tool, not a toy. I like to bring it with me incase the cops do show up. That way I can prove that I'm inocent. (Much against common misconception, those of us who work for the government are not imune to the law!) ;)

I appoligize for any spelling errors, I don't have spell check on my droid, and I'm a terrible speller.